Method for continuously growing epitaxial layers of semiconductors from liquid phase

ABSTRACT

SINGLE-CRYSTAL EPITAXIAL LAYERS OF SEMICONDUCTORS ARE GROWN ON SUITABLE SUBSTRATES FROM THE LIQUID PHASE. THE SUBSTRATES ARE MOVED CONSECUTIVELY ON A PLURALITY OF VESSELS IN A REACTION FURNACE FROM A HIGH TEMPERATURE REGION TO A LOW TEMPERATURE REGION MAINTAINED WITHIN THE REACTION FURNACE, WHICH IS PROVIDED WITH A TEMPERATURE GRADIENT IN THE LONGITUDINAL DIRECTION, ALL THE WHILE THE SUBSTRATES ARE KEPT IN CONTACT WITH THE LIQUID SOLUTION CONTAINING A SOURCE MATERIAL OF THE EPITAXIAL LAYERS, WHEREBY THE EPITAXIAL LAYERS ARE GROWN SUCCESSIVELY AND ON A CONTINOUS BASIS.

May 7, 1974 SHN-16H1 AKAl ETAL 3,809,584

METHOD FOR OONTINUOUSLY GROWING EPITAXIAL LAYERS OF SEMICONDUCTORS FROMLIQUID PHASE 5 Sheets-Sheet 1 Filed Jan. 18, 1973 wk O O Or May 7, 19743,809,584

METHOD FOR CONTINUOUSLY GROWING EPITAXIAL LAYERS OF SEMICONDUCTORS FROMLIQUID PHASE Filed Jan. 18, 1975 3 Sheets-Sheet 2 May 7, 1974SH|N|CH|AKA| ET AL 3,809,584

METHOD FOR CONTINUOUSLY GROWING EPITAXIAL LAYERS OF SEMICONDUCTORS FROMLIQUID PHASE Filed Jan. 18, 1973 3 Sheets-Sheet E,

FZ//QA/A cih/516,47'

72 77 Eupen/m95 United States Patent Oce 3,809,584 Patented May 7,

Im. c1. H611 7/38 U.S. Cl. 148-172 8 Claims ABSTRACT oF THE DISCLOSURESingle-crystal epitaxial layers of semiconductors are grown on suitablesubstrates from the liquid phase.

The substrates are moved consecutively on a plurality of lvessels in areaction furnace from a high temperature regionto a low temperatureregion maintained within the reaction furnace, which is provided with atemperature gradient in the longitudinal direction, all the while thesubstrates are kept in contact-with the liquid solution containing asource material of the epitaxial layers, whereby the epitaxial layersare grown successively and on a continuous basis.

BACKGROUND OF THE INVENTION The present invention relates to a methodfor successively growing liquid phase epitaxial layers which enables theepitaxial growth of semiconductors on a commercial basis.

As to techniques of growing epitaxial layers on singlecrystal substratesfrom the liquid phase, there has been widely employed the tiltingtechnique (Nelson method), dipping technique, sliding technique, etc.These techniques comprise the so-called slowly-cooling liquid phaseepitaxial growth method, wherein the single-crystal substrates arecontacted with the liquid solution containing a source material and thenthe temperature in the reaction furnace is lowered at a suitable rate.In those cases where the liquid phase epitaxial layers are to be grownon many substrates on a commercial scale according to the conventionalslowly-cooling liquid phase epitaxial growth method, the temperature ofthe reaction furnace after completion of each growth operation has beenreduced to a point that is lower than the initial temperature by apredetermined number of degrees and, accordingly, the temperature in thereaction furnace must be elevated again to the initial temperature pointbefore performing the next operation. Consequently, the epitaxial growthis inevitably not accomplished on a truly continuous basis and isinefficient, since a time is required for elevating temperature betweena growth operation and the next growth operation. For a detaileddescription, see H. Nelson, Epitaxial Growth From the Liquid State andIts Application to the Fabrication of Tunnel and Laser Diodes, RCAReview 24, p. 603, 1963, or see U.Sl. Pat. No. 3,565,702. Thesedifficulties are inevitable in the liquid phase epitaxial growth method.'Ihe liquid phase epitaxial growth method has not been suitable enoughfor the production on a commercial basis.

SUMMARY OF 'II-IE INVENTION The present invention relates to animprovement in or relating to said slowly-cooling liquid phase epitaxialgrowth method. By the method of this invention, epitaxial layers of thefollowing various semiconductors can be grown successively on suitablesubstrates: Group III- V compound semiconductors such as GaAs, GaP, InP,

InSb, InAs4 and GaN; Group III-V mixed semiconductors such as Ga1xAlXAs, InAsLXSb,' In'xGalQxP, In`Ga1 ,Sb, InxAl1 XP, GaxAl1 XP and In1xAlXAs (0 x` 1 in the foregoing semiconductors); Group II-VI compoundsemiconductors such as CdTe, CdS, ZnSe, ZnTe, ZnOy and BeTe; mixedcrystals of Group II-VI semiconductors such as ZnSe1 xTex, Cd1 ZnXS andHg1 XCd,Te (0 x 1 in the foregoing semiconductors) and other compoundsemiconductors such as (ZnS)1 X(GaP)X, (ZnSe)1 (GaAs)x (0 x 1 in thevforegoing semiconductors) and ZnSip2.

According to the method comprising this invention, a suitable substrateconsisting of a single crystal of a semiconductor is moved through areaction furnace having a temperature gradient from a high temperatureregion to a low temperature region within the reaction furnace, all

the while the substrate is kept in contact with the liquid.

solution in which a source material of the 'single-crystal epitaxial`layer of the semiconductor to be grown is -dissolved, thereby growingthe epitaxial layers successively.

Thus, the principal object of this invention is`to provide a method ofgrowing liquid phase epitaxial` layers of semiconductors successively,as well as continuously, on many suitable substrates from the liquidphase.

A feature of thisinvention resides in a method'of grow` ing epitaxiallayers of semiconductors on suitable singlecrystal substrates from theliquid phase, wherein the single-crystal substrates are moved through areaction furnace from a high temperature region to a low temperatureregion within the reaction furnace, which is provided to have atemperature gradient in the longitudinal direction thereof, all thewhile the substrates are kept in contact with the liquid solutioncontaining a source material of the semiconductor, thereby growingepitaxial layers of said semiconductors on said single-crystalsubstrates.

Another feature of this invention resides in a method of growingepitaxial layers of semiconductors on suitable single-crystal substratesfrom the liquid phase wherein substrate-supporting vessels are employedwith each containing at the bottom of the vessel the single-crystalsubstrate and a liquid solution containing a source material of thesemiconductor is supplied onto the substrate in thesubstrate-'supporting vessels from a main container of liquid solutionhaving at least five times as much Volume as the vessel. The vessels aremoved through a reaction furnace from a high temperature region to a lowtemperature region within the reaction furnace, which is provided tohave a temperature gradient in the longitudinal direction of thefurnace, all the while the single-crystal substrates are kept in contactwith the liquid solution, thereby growing epitaxial layers of thesemiconductors on the single-crystal substrates.

Still another feature of this invention resides in a method of formingepitaxial growth layers of semiconductors on suitable single-crystalsubstrates from the liquid phase wherein substrate-supporting vesselsare employed with each containing at the bottom of the vessel thesingle-crystal substrate and these vessels are vertically moved in avertically elongated reaction furnace having a vertical temperaturegradient, from a high temperature region to a low temperature regionwithin the reaction furnace, all the while the single-crystal substratesare kept in contact with a liquid solution containing a source materialof the semiconductor so that the single-crystal substrates in the vesselare positioned on the low temperature side of the reaction furnace andthe solution is positioned on the high temperature side of the furnace.

BRIEF DESCRIPTION OF I'HE DRAWINGS The foregoing and other objects,features and advantages of this invention will be apparent from thefollowing more particular description of preferred embodiments 3 of theinvention as illustrated in the accompanying drawings.

In the drawings:

FIG. 1 illustrates in cross section a horizontal reaction furnace orapparatus for continuously growing the epitaxial layers on suitablesubstrates from the liquid phase together with a graphic illustration ofthe longitudinal temperature profile of the reaction furnace.

FIG. 2 is a perspective view of a portion of the apparatus shown in FIG.1.

FIG. 3 illustrates in cross section a vertical reaction furnace rapparatus for continuously growing the epitaxial layers on suitablesubstrates from the liquid phase together with a graphic illustration ofthe vertical temperature profile of the reaction furnace.

DESCRIPTION OF PREFERRED EMBODIMENTS THROUGH EXAMPLES The following areexamples of employing the method and apparatus comprising this inventionfor growing the single-crystal epitaxial layers of semiconductors onsuitable substrates by the continuous liquid phase epitaxial growthmethod.

Example 1 FIG. 1 is a cross sectional View of a liquid phase epitaxialgrowth reaction furnace apparatus for carrying out the continuous liquidphase epitaxial growth used in this example together with anillustration of the temperature profile in the reaction furnace. FIG. 2is a perspective view of a portion of the inside of the reaction furnaceshown in FIG. 1.

In FIG. l, a main container 2 of a liquid solution 1 containing a sourcematerial is supported from within the reaction tube S and is kept at atemperature T1 by an electric heater 9. In the main container 2, theliquid solution 1 in which a source material consisting of constituentelements or the semiconductor has been dissolved is contained. A frame 4for supporting the liquid solution is positioned upon a slider orcarrier 3 to provide a plurality of substrate-supporting vessels 20. Atthe bottom of each vessel 20 or on the slider surface, a single-crystalsubstrate 5 is positioned. The substrate-supporting vessels 20containing the single-crystal substrate 5 are continuously moved in thedirection of an arrow 19 into the reaction furnace heated by an electricfurnace and thereafter introduced into the lower portion or area of themain container 2.

An opening 11 is provided in the bottom of the main container 2 throughwhich the liquid solution is supplied into each vessel 2t) passingtherebeneath. The main container 2 is provided with at least tive timesas much volu-me as a single vessel in order to feed the liquid solutionsuccessively into a plurality of vessels 20 on a continuous basis. Onthe right side of the container 2, a temperature gradient of 0.1-15C./cm. is provided horizontally through the furnace by an electricheater 12 as depicted in the temperature profile curve at 21. Thetemperature gradient through the furnace is controlled depending uponthe desired conditions of the epitaxial growth and moving rate of thevessels 20 through the furnace. It is desirable to select the value ofthe product GV C./min.) of the temperature gradient G C./cm.) and themoving rate V (cm/min.) in the range of 0.1-10 C./min. Temperaturegradient in the vertical direction through each vessel 20 is notnecessarily required but preferably the temperature of the singlecrystalsubstrate 5 is lower than that of the solution. While each vessel 20containing the single-crystal substrate 5 and the deposited liquidsolution is moved through a remainder of the furnace, an epitaxial layeris grown on the single-crystal substrate 5. The right side or area ofthe reaction tube 8 is kept at a predetermined temperature T2 by anelectric heater 13. Each vessel 20 is then separated into the frame 4and the slider 3, whereby the liquid solution is removed from thesurface ofA the epitaxial layer grown on the single-crystal substrate.5. By providing the proper temperature T2, the epitaxial layers aresubjected to suitable heat treatment for annealing purposes. FIG. 2illustrates an embodiment for separation of the frame 4 from the slider3. As the vessel 20 moves along, the frame 4 for supporting theliquidsolution is caused to slide by a xed arm member 14 intherdirection of an arrow 22 which is substantially perpendicular to thedirection of vessel movement depicted by Varrow 19, so that the liquidsolution is separated by member 14 from the surface of the epitaxiallayer grown on'the single-crystal substrate 5.

In employing the method of this invention, a solid or integralsubstrate-supporting vessel 20 may also be used. In this case, theliquid solution is .removed after the entire vessel has been taken outfrom the reaction tube.

In FIG. l, hydrogen gas or an inert gas such as nitrogen gas isintroduced through a gas inlet 6 of the tube 8, and the gas is exhaustedthrough a gas outlet 15. Hydrogen gas or an inert gas such as nitrogenis introduced at outlets 7 and 16 and exhausted, respectively, throughoutlet 17 and outlet 18 of the reaction tube 8. Through the .employmentof this construction, the absence of an air atmosphere within thereaction tube 8 is maintained.

A semicontinuous liquid phase growth method is also contemplated withinthe scope of this invention wherein the temperature region of T1 islonger than that shown in FIG. 1 because the quartz reaction tube 8 islonger than that shown in FIG. 1. Also, the main container 2 would bepositioned at the left end of the tube to supply the liquid solutioninto each vessel 20 which are fixed so that the reaction furnace itselfis moved-in a direction toward the container 2, that is, toward the leftof FIG. l.

In this example, GaP epitaxial layers were grown successively on GaPsingle-crystal substrates from the liquid phase. In FIG. 1, the liquidsolution 1 comprises about 30 g. of Ga and 1.0 g. of GaP. Temperature T1of the solution was kept at 1050 C.`In the formation of nepitaxialgrowth layers, Te or S was doped as impurity. As the single-crystalsubstrates 6, wafers obtained from GaP single crystals grown bytheliquid encapsulated Czochralski process were used. Surface area ofthe'wafer was about 2 cm?.

The single-crystal substrate 5v was coveredk 'withmthe liquid solutionto a depth of about 2 mrn. as supported within the frame 4 which isabout 2 mm. in'height'for supporting the liquid solution. The substrateSand the liquid solution were moved in this state in the 'reaction tube8 having a temperature gradient of 2 C./cm. at a rate of 3 cm./min. inthe direction of arrow 19. At the growth completion portion of thefurnace,i the temperature is kept at T 2=950 C. Finally,`frame 4 isseparated from the moving slider 3 by means of the xed arm member 14.Thus, there is obtained epitaxial growth layer with a thickness of about30 um.' Generally, the height of the frame may be selected inthe rangeof 0.5"- 5 mm. The depth of th'e liquid solution over the substrate 5may be 0.5-5 mm., depending upon the height of the frame 4. If the depthis too small, the epitaxialgrwth is diflicult. Too large of a'depth isundesirable from 'an economicalviewpoint. It is desirable to provide asolid cover (not shown in the figure) on the'liquid solution jso thatthe depth of the liquid solution is uniform.

In growing the liquid phase epitaxial layers of the III-V compoundsemiconductors and mixed crystal semiconductors, volatile components ofthe semiconductorsor volatile doping impurities may be evaporated out ofthe solutions. In such a case, the loss from the solution by evaporationmay be covered up by putting a lid on the top of container 2 and alsoeach vessel 20 to prevent the evaporation or may be compensated byintroducing the volatile compound into the reaction tube 8 together withthe carrier gas through the inlet 6. Moreover, it is possible tocounter-dope an impurity of the different type by in;

troducing the impurity in vapor form, thereby growing the epitaxiallayers containing a p-n junction.

Reference is made to FIG. 3 which illustrates in cross section avertical reaction furnace for continuously growing the epitaxial layersused in this particular example together with the temperature profilevertically through the reaction furnace. This process in which thevertical reaction furnace is used is superior to that in Example 1wherein the horizontal furnace is used for` the following reasons:

(1) In the process in which the horizontal furnace is used, thetemperature gradient is also produced on the surface of thesemiconductor substrateand convection of the solution is produced sincethe semiconductor substrate is placed horizontally in thesubstrate-supporting vessel and is in contact with the solution in thereaction furnace having a horizontal temperature gradient. Therefore,the temperature across the deposited solution in the vessel isnotuniform. On the other hand, according to the process in whichthevertical reaction furnace is used, the direction of the temperaturegradient is perpendicular to the surface of the semiconductor substrateand the temperature on the surface of the semiconductor substrate isuniform and, therefore, more uniform epitaxial growth layer :can beobtained.

(2) In the liquid phase epitaxial growth, it has been known that for.obtaining uniform epitaxial layers, conditions are selected so thattemperature of the semiconductor substrate is lower than that of thesolution, as, for

example, the substrate is cooled with a special cooling device. For thispurpose, it is not desirable that temperatures of the semiconductorsubstrate and the saturated lsolution be the same. The conditions can besatisfied by l(3) In effecting the successive epitaxial growth, the

semiconductor substrate wafers are naturally arranged in the horizontaldirection if the horizontal furnace is used. However, temperaturegradient in each semiconductor substrate mustbe minimized and,consequently a sharp temperature gradient 'should not be produced.Therefore, for providing a necessary temperature gradient between thelowtemperature regionl andV the high` temperature region, a very longfurnace must be used. On the other hand, in the vertical reactionfurnace, temperature on the surface of each semiconductor substrate isuniform and a sharp temperature gradient in the direction perpendicularto the wafer surface can be obtained and, consequently, the furnace of asmall size and length may be used. In the case of the vertical furnace,it is desirable to select the temperature gradient G C./cm.) in therange of l,-50 C./cm.

As shown in PIG. 3, a substrate-supporting vessel 25 made of carbon,quartz or BN is provided for containing a semiconductor substrate 23 anda solution 24 saturated Withthesource material. An elevator conveyor isprovided to have many shelves for lowering the individual vessels 25 tothe lower portion of the vertical furnace 43. Heaters 27 through 41 and52 are provided in the furnace 43. The T-shaped quartz tube 42 is housedwithin the internal area of the furnace 43. The overall length of thefurnace 43 is about 1 m. The temperature gradient is produced so thathigh temperature (T1 C.) region is :positioned in the upper portion ofthe furnace 43 and low temperature (T2 C.) region is positioned in thelower portion thereof, with the middle portion adapted to have nearly astraight temperature gradient (see temperature profile 51 on the left ofFIG. 3). The temperature gradient, T1 and T2 can be changed selectivelydepending upon variety ofthe compound semiconductor to be epitaxillygrown, and thickness of theepitaxial layer to be grown. Each vessel 25having a frame for supporting the liquid solution 24 and containing theGaP semiconductor substrate 23 is introduced in the direction of anarrow 44 beneath the lower or bottom inlet 47 of a main container 46containing a Ga solution saturated with GaP source material. The Gasolution 24 saturated with GaP is fed into the frame of the vessel 25and then introduced into the conveyor 26. In this particular example,the GaP semiconductor substrate 23 had a thickness of 450 am. and thesurface area on which the epitaxial layer is to be grown was about 2om?. The liquid solution 45 comprised about 30 g. of Ga and 1.8 g. ofGaP. The solution 45 was kept at a temperature T1 of 1050 C. In theformation of n-epitaxial growth layer, Te or Ga2S2 was added asimpurity. Temperature gradient in the furnace 43 was about 2 C./ cm. Thelow temperature region in the lower portion of the furnace was kept atabout 950 C Each vessel 25 was lowered at a rate of 3 cm./min. from thehigh temperature region to the low temperature region in the furnace 43,which took about 20 minutes. The vessel 25 lowered to the lowtemperature region was then moved in the direction of an arrow 48 bymeans of a push bar 49. The frame for supporting the solution of thevessel 25 was removed by a jig bar 50 having the same structure as thearm member 14 in FIG. 2. Thickness of the resulting epitaxial growthlayer was about 30 nm.

Although temperature gradient in this example was nearly straight, thetemperature profile may be produced so that the temperature gradient ina high temperature region changes slowly and the temperature gradient ina low temperature region changes sharply. Further, the middle part ofthe temperature prole may be kept at a temperature l0-50 C. higher thanthe high temperature region and then the temperature may be madegradually lower thereafter in order to dissolve 1-5 um. thickness of thesurface of the semiconductor substrate 23 in the solution 24 and bringabout epitaxial growth.

While this invention has been particularly shown and described withreference to preferred embodiments thereof, it will be understood bythose skilled in the art that the foregoing and other changes in formand details may be made therein without departing from the spirit andscope of the invention.

We claim:

1. The method of continuously growing epitaxial layers of semiconductorson single-crystal substrates from the liquid phase comprising the stepsof moving a plurality of consecutively arranged vessels through areaction furnace provided with a high temperature region and a lowtemperature region, each of said vessels including a single-crystalsubstrate, depositing the liquid solution containing a source materialof the semiconductor within each of said vessels on the surface of thesubstrate as they proceed through the furnace within the hightemperature region, and thereafter continuously moving each of saidvessels from the high temperature regionv through a decreasingtemperature gradient region and thence into the low temperature regionof the furnace while said substrates are maintained in contact with theliquid solution thereby growing epitaxial layers of said semiconductorsconsecutively on said substrates.

2. The method of claim 1 characterized in that said semiconductor is aGroup III-V compound semiconductor.

3. The method of claim 2 characterized in that said Group III-V compoundsemiconductor is GaP.

4. The method of claim 1 characterized in that said liquid solution incontact with said substrate has a height ranging from 0.5 to 5 mm.

5. The method of claim 1 characterized by the step of separatingl theliquid solution from each, of the epitaxial layers grown on saidsubstrates as said vessels move through said low temperature region ofsaid furnace.

6. The method of claim 5 characterized by providing van inert atmospherewithinfthe interior of the furnace.

7. The method of claim 1 characterized bygco-ordinat- Iing thetemperature gradient in the furnace with the rate of movement of said'vessels therethrough thereby controlling the nature and rate ofepitaxial growth.

8. The method of continuously growing epitaxial layers of semiconductorson `single-crystal Isubstrates from the liquid phase vcomprising thesteps of moving a plurality of consecutively arranged vessels 10 througha vertical reaction furnace provided with an upper high temperatureregion and a lower low yternperature region, each of said Vesselsincluding` a single-crystal substrate, I

` depositing the liquid solutioncontaining a source ma- 15 terial ofthe'semiconductor within each of saidvessels on the surface of thesubstrate as they proceed through the high temperature region, and

continuously but uniformly moving each of said vessels consecutivelyfrom the high temperature region 20 downwardly through a decreasingtemperature gradient'region of the furnace and thereafterliinto said lowtemperature region while said substrates are maintained in contact with'the' liquid-solt'ion y'thereby growing epitaxial layers 0f Said'slen'iicor'b'y `A `ductors on said substrates.

i References Cited UNITED STATES PATENTS 3,e15,944

10/11971 Sheng et al 148,-18-9 13,163 1,836 1/1972 Jarvela et al 14S-171 X .3,665,888 5/ 1972 Bergh et al. 148-,1711.X-

3,690,965 9/ 1972 Bergh et al. f1,48,- 1-7,2

Solomon 148-.-7-1-71 GEORGE. T. OZAKI, Primary Examiner U.S. Cl. X.R.

117-201; HSL-415; 148-17*1, 173

